1. Field of the Invention
The present invention relates to a method and apparatus for determining the deformation which occurs on a surface of an object as a result of stressing the object, and more particularly pertains to the invention of a rapid non-destructive, non-contacting device for determining strain and stress in developmental or operating systems, such device utilizing the principle of laser speckle interferometry or an extension of laser speckle interferometry as the device has these alternative capabilities and both are innovations and claims of this patent. The extension of laser speckle interferometry is itself an entirely new concept that is not interferometric in nature but rather a recording, digitizing, correlation and analysis of laser speckle patterns to yield strain and stress. The new concept, although not interferometric in nature, is an extension of the laser speckle interferometric technique in that the same mathematical basis for analysis is employed.
2. Description of the Prior Art
The determination of strain and stress in structural members or in pressure vessels is a problem that is of interest to those associated with structural or pressurized systems. The design engineer or stress analyst is faced with predicting the performance of a system, while the operating engineer or plant supervisor is concerned with monitoring an operating system to insure structural integrity during normal or emergency situations. By the same token, the prototype test engineer is in the middle of the spectrum and is faced with evaluating experimentally the predictions of the design engineer or stress analyst, and eventually certifying the system for operation. The available tools for stress and strain determination, either analytical or experimental as appropriate to each case mentioned, are inadequate or present difficulties to the user.
The prototype test engineer, and the operating engineer or plant manager face additional difficulties if the system operates in or creates a hostile environment of high temperature, toxicity or nuclear radiation, or is in a state of pending disaster. The prototype or operating engineer in these situations is dealing with full sized systems inaccessible to man and intolerant to strain monitoring devices that physically contact the system. This type of situation in many cases, requires or could benefit from continuous real time image data on the strain condition of the system. In this respect, strain and stress condition monitoring is required routinely as evidenced by an article in the Wall Street Journal of June 22, 1977 which states, "it (Nuclear Regulatory Commission) will require inspection of most of the commercial nuclear reactors in the U.S. because cracks have been found in cooling pipes in six units".
With regard to pressure vessels and pipes operating in hostile environments of elevated temperature and nuclear radiation, it should be noted that the same may be subjected to temperature and pressure excursions and cycling which in turn indicates the possibility of thermal stresses. In this respect, the determination of thermal stresses is not easily accomplished, since there are at least three classes of thermally induced stresses.
Specifically, thermally induced stresses may include shrinkage stresses, temperature gradient stresses and thermal shock stresses. In this connection, shrinkage stresses are induced in a body when the body is restrained from free expansion or contraction during a uniform temperature change. Problems involving shrinkage stresses arise in several different areas such as the fabrication of composite structures which have different coefficients of thermal expansion for each material and the operation of energy conversion systems where boundary constraints are imposed against free expansion of a homogeneous structure.
Similarly, temperature gradient stresses are induced in a body due to temperature gradients within the body. These temperature gradients can be of a steady state or transient nature. In either case, the expansion or contraction of the body is not uniform within the body and the resulting internal forces give rise to strains and stresses. When steady state gradients exist, stresses arise if the body is multiply connected as opposed to being simply connected. Multiply connected bodies include plates with holes, turbine blades, heat exchanger tubes and nuclear fuel elements. Transient temperature gradients give rise to stresses in both simple and multiply connected bodies. Usually, the induced stresses associated with transient gradients vary with time until an equilibrium condition is reached. The determination of stresses induced by transient gradients are extremely difficult to solve analytically and in fact, experimental techniques offer the best analysis of stresses induced by transient thermal fields.
As to thermal shock stresses, these arise when a body is subjected to an instantaneous thermal gradient such as a sudden change in surface temperature. The resulting stresses are in essence stress waves that are set up in the body much the same as shock waves. This type of stress occurs when the temperature change in time is considerably less than the mechanical response time of the solid body.
It should be noted that stresses due to transient thermal gradients and thermal shock are often at levels above the elastic limit. Thus, the end results can be severe and conditions such as creep, buckling, collapse, and low thermal fatigue are not uncommon. The experimental method employed to analyze bodies subject to the stresses arising from the three thermal conditions above-described include photothermal elasticity, scattered light photoelasticity, and holographic interferometry.
Photothermal elasticity investigations of thermal stresses have provided experimental solutions to a broad class of two and three dimensional problems. Basically, the methods employed rely on the use of models of the body under study and have been confined to the study of the effects of thermal stresses only. The results of these studies are subject to discussion since the material properties (modulus of elasticity and coefficient of thermal expansion) of the model and the prototype are different and hence, the thermal stresses must be separated from external loads. Also, the available range of temperatures for photothermal elasticity is very small compared to the possible temperature effects in prototypes. Therefore, solutions usually are obtained by solving separately the thermal stress problem and the external load problem, and then superimposing the results. While this experimental technique has provided solutions to a broad class of problems, the limitations to actual prototype measurements are obvious. In this regard, photothermal elastic techniques include the two dimensional model technique, the stress freezing technique or the embedded polariscope technique.
With respect to the two dimensional model technique, two dimensional solutions involve either plane stress or plane strain assumptions. This solution technique was first applied as early as 1935 for steady state thermal conditions and models. As to the stress freezing technique, there have been several variations of this technique employed in thermal stress problems. However, this classical method is not applicable to problems which require a thermal gradient to be maintained in a model. This is because the optical and physical properties change drastically around the critical temperature (the temperature at which stresses are optically frozen in the model material). Furthermore, this experimental method is not applicable to mixed boundary value problems. These problems arise when two boundaries which have different coefficients of thermal expansion are fixed.
Probably the most useful technique for performing photoelastic studies of thermal stresses involves the use of an embedded polariscope to isolate a plane of a three dimensional model. Embedded polariscopes are particularly applicable to thermal gradient problems and to the determination of stress concentrations due to thermal gradients. However, while this technique is the most promising of the classical photoelastic methods, the basic limitations still apply. Also, the investigator must know a priori which embedded plane to isolate for investigation.
Scattered light photoelasticity is the only one of the classical methods which offers a truly three dimensional solution technique. A photoelastic model is illuminated with a laser and planes are observed in much the same manner as in the embedded polariscope technique. The principal advantage of scattered light photoelasticity is that planes internal to a body can be isolated and stresses resulting from thermal gradients and mechanical loads can be studied. This is the only photoelastic method which is non-destructive and the model can be utilized for many loading configurations. Of all the photoelastic techniques, this method represents the most general solution method for three dimensional transient thermal stress applications. However, the general limitation of all photoelastic studies applies also to the scattered light technique.
Holographic interferometry has been used to study the displacement in high temperature fields in prototypes. These studies represent feasibility investigations, and they have established the potential of the technique for studying thermally induced stresses. However, the technique requires additional research to establish experimental procedures that yield reproducible and accurate results. The principal advantage of holographic interferometry is the fact that it is a very sensitive method of measurement, and the whole surface of a structure can be investigated at the same time, rather than on a point-by-point basis. Although holographic interferometry is well suited for measuring normal movements, there is no general way of eliminating the effect of normal movements from in-plane movements. Also, holographic techniques possess the disadvantage of requiring several separate views of the holographic fringe patterns of a surface. This requires a considerable amount of data reduction to separate out the in-plane displacement field. While holographic interferometry is recognized as an effective optical technique for employing a coherent light source (laser) for detecting and measuring the components of surface displacement and strain, so as to permit a determination of the entire displacement or strain field, there is an additional optical technique known as laser speckle interferometry which presents a promising means of determining stress and strain in bodies.
Laser speckle interferometry is the most recent advance in coherent optics used in an engineering application to measure stresses and strains in bodies, and shows promise of alleviating many difficult problems in experimental mechanics. The basic method utilizes simple high-resolution photographs of a surface which is illuminated by coherent light. The result is a real time or permanently stored whole-field record through interference fringes of a deformed surface. This record yields a map of displacements in the object. On the basis of these introductory statements relative to laser speckle interferometry, some specific examples of the application of the principle are presented. Suggestions for the direct use of coherent light in displacement metrology and contour mapping first appeared in 1968, and it has been shown that if two identical speckle patterns are superimposed on a photographic plate translated laterally by a short distance between exposures, then the diffraction halo generated by the processed plate will consist of a pattern of parallel straight fringes. The diffraction halo observed through a small area of the recorded image will correspond to the local displacement at the corresponding point on the object, and the direction of the fringes will be orthogonal to the direction of the local displacement vector. Additionally, by optically illuminating the developed photographic plate with a converging spherical wave, the entire surface can be analyzed at one time to determine the displacement field of the surface.
In addition to the above, another technique utilizes the laser speckle effect for measuring either normal or in-plane components of displacement over an entire surface at one time. Discussion of in-plane measurements follow to illustrate the general approach. For measurement of the in-plane components of displacement, a surface is illuminated by two beams of coherent laser light, symmetrically disposed about the normal to the surface. These two speckle patterns are superimposed and their resultant speckle pattern is recorded on film. The intensity distribution of the resultant speckle pattern depends on the relative phase of the component patterns. Then one or both speckle patterns is changed and again, the resultant speckle pattern is recorded on the same photographic film. By measuring correlation between the resultant pattern at two different times, a change of relative phase is detected, which in turn gives a measure of surface displacements. These correlation fringes are observed either in real-time or by combining two transparencies having resultant speckle patterns at two different times and illuminating the pattern in a Fourier filter system. A major drawback of this technique is that the path length difference between the two illuminating beams has to be less than the coherent length of the light used to generate correlation fringes.
Two different variations of the dual beam approach for measuring in-plane surface displacements exist. In the first method, the displacement is determined by photographing a coherently illuminated object through two laterally displaced apertures. The displacement is displayed as a pattern of Moire' fringes over the image of the surface. Thus, there is no need for scanning of the beam on a point-by-point basis. As the surface is illuminated by only a single laser beam, the implementation problems associated with the dual-beam technique (mechanical stability and equal path lengths between the various optical components) are minimized. In the second method, the object is illuminated using a single laser beam and photographed via a double exposure before and after displacement. The Fourier transform of the doubly exposed transparency is obtained optically by illuminating the photographic plate with a converging spherical wave. The main advantage of this procedure is that the whole-field displacement can be analyzed and, by appropriate position of a set of apertures in the transform plane, any component of the displacement normal to the line of sight can be detected with variable sensitivity.
The main advantages of speckle interferometry over photoelasticity and holographic interferometry include the fact that many measurements may be obtained within the confines of the laser photograph which, with powerful pulsed lasers, could cover several meters of surface area. Further, the line of movement and its components of motion are given by the fringes, and strain gages which are susceptible to damage are not utilized. Additionally, the speckle interferometry method is non-contacting, and no special cleaning or surface preparation is necessary, with the exception that only contamination such as thick dust must be removed. Finally, measurements at high temperatures far in excess of the strain gage range are possible.
Speckle interferometry does have some limitations however, such as the fact that measurements are not as accurate as those made with strain gages. However, the inaccuracies usually associated with the laser speckle technique involve numerical error in calculating the derivatives, and not the metrology of the laser speckle technique per se. Strain gage measurements with accuracies of 1% can be obtained while strain calculations using the speckle data technique result in accuracies of approximately 5%. The accuracies associated with the numerical analysis technique used with the speckle approach will be improved as a result of the computerized approach employed by the present invention. Also, fringe analysis is time consuming in that the analyst must view the photographic plates, make the necessary measurements and then calculate the results. However, this limitation can be overcome by using an optical data digitizer system, including an image storage device and a computer for data correlation and analysis, as proposed by the present invention.
In summary, it should be pointed out that the classical techniques employed to determine thermal stresses must rely on the use of models of the operating system being investigated. Laser speckle interferometry is not dependent on the use of models and is in fact applicable to full scale systems. Many of the recent advances in coherent optics have suggested the engineering applications to prototype systems; however, with all of the techniques developed thus far, the recording medium has been photographic film. Therefore, data analysis has required the use of a specialist for interpretation. Even with this limitation, though, the great potential of coherent optics techniques for engineering analysis has been clearly demonstrated.
The laser speckle effect, which is the basis of the new concept of this invention, promises to be the optical technique whereby the photographic film can be eliminated in the data acquistion process. The elimination of the photographic film means that interference fringe patterns classically associciated with laser speckle interferometry need not be employed to yield data. These fringe patterns can be generated in the electronic imaging system of this invention and analyzed as in the classical case of interferometry and this capability is one claim of the invention. However, the invention is cabaple of eliminating this step by introducing a laser speckle technique based on the digital correlation of successive laser speckle patterns before and after object surface deformation. Thus, laser speckle and digital correlation, as the technique is proposed to be termed, has its foundation in laser speckle interferometry and has all the attributes of laser speckle interferometry without the necessity of photographs and fringe measurements. The laser speckle and digital correlation will be accomplished through the development of electronic video systems capable of high resolution of the speckle patterns. The compatibility of the speckle technique with image processing offers a user-oriented system for a wide range of engineering applications. Finally, it should be pointed out that there is commercially available an Electronic Speckle Pattern Interferometry (ESPI) device for time averaged holographic stress analysis, as reported in Materials Evaluation (May, 1979). However, this device, while demonstrating the fact that speckle patterns can be digitized, does not include the use of a computer for data analysis and management and does not have the capability to by-pass the step of creating interference fringe patterns, as its sole purpose is to create these patterns. Furthermore, the ESPI device requires that the body under investigation be vibrated in order to generate the fringe patterns as opposed to illumination alone by a coherent light source.